1. Field of the Invention
This invention relates to a method for obtaining a wide electrochemical window in molten salts particularly in chloroaluminate molten salts.
2. The Prior Art
Chloroaluminate molten salts feature a range of Lewis acidity comparable to Bronsted acidity present in water. Just as pH greatly affects the chemistry and electrochemistry of aqueous solutions, the chloroacidity is a major determinant in speciation, reactivity and electrochemistry in chloroaluminates. Chloroaluminate molten salts are mixtures of aluminum chloride and a chloride donor, which is usually an alkali metal chloride or organic chloride. The most studied chloroaluminate molten salts are the mixtures NaCl/AlCl.sub.3, BPC/AlCl.sub.3 and MEIC/ALCl.sub.3, where BPC is 1-(1-butyl) pyridinium chloride and MEIC is 1-methyl-3-ethylimidzolium chloride. The latter two binary molten salts have the advantage of having compositions with melting points below room temperature, i.e. they are room temperature molten salts or ionic liquids.
Chloroaluminate molten salts with melting points below room temperature are attractive candidates for battery electrolytes. In one example, 1-methyl-3-ethylimidazolium chloride and aluminum chloride, herein MEIC-AlCl.sub.3, many of the physical, chemical and electrochemical properties of the molten salt, depend on the composition, i.e., the relative proportions of MEIC and AlCl.sub.3. The maximum voltage that a battery cell can deliver is determined by the decomposition potentials of the electrolyte. The difference between the anodic and cathodic decomposition limits is called the electrochemical window of the electrolyte.
The window in the MEIC-AlCl.sub.3 melts depends on the chemical species present, which is determined by the composition. The value for the molten salts is about 2.4 V, compared with about 1.5 V for water based electrolytes. While 2.4 V is a good window, the molten salts have a particular composition where the window expands to 4.5 V. Unfortunately that point, with the wide window, is where the MEIC and AlCl.sub.3 are equal, at 50 mole fraction each (called the neutral point) and variations from such equivalence, can significantly reduce the 4.5 window. However, keeping a neutral composition is like balancing on a knife edge. For example, any chemical or electrochemical process that consumes or generates acidic or basic species will remove the melt from its neutral composition.
What is needed is a method to maintain such neutrality and such wide window despite the above chemical activity in chloroaluminate molten salts. However such method has not previously been available.
In related prior art, U.S. Pat. No. 4,463,071 to Gifford et al. (1984) relates to the use of chloroaluminate molten salts as electrolytes with the addition of alkali metal salts, including sodium chloride, to such electrolytes. However, the Gifford reference discloses such salts as a source of cations for intercalation into polymer or other electrodes during the cell operation (Col. 2 lines 18-21 and Col. 6, lines 15 and 16). That is, Gifford's cathodes will not function without such cation supply.
The present invention however, adds alkali metal chloride to chloroaluminate molten salts for a different purpose, to buffer them, as more fully discussed below.
Accordingly, there has now been discovered, a method for obtaining a wide electrochemical window in chloroaluminate molten salts by adding a reagent that acts as a Lewis buffer for neutral composition in such molten salts. A Lewis buffer is a material that prevents or reduces changes in the Lewis acidity or basicity of a solution. In the case of chloroaluminate molten salts, the buffer must affect the chloride (Cl-,the Lewis base) and the heptachloroaluminate (Al.sub.2 Cl.sub.7 -, the Lewis-acid) and keep the concentrations thereof very low.